1. Field of the Invention
The present invention relates to an apparatus for controlling an operation of a linear compressor, and more particularly, to an apparatus for controlling an operation of a linear compressor by which an unstable phenomenon caused due to a characteristics deviation of parts of a compressor is corrected to stabilize the operation of the system, thereby accomplishing an optimal operation, and to its method.
2. Description of the Background Art
A linear compressor is driven by a linear oscillating motor, without requiring a crank shaft which changes a rotational movement to a linear movement, so that there is little frictional loss. For this reason, the linear compressor is known to have a high efficiency compared to any other compressor.
Moreover, where the linear compressor is used for a refrigerator or an air-conditioner, since a compression ratio thereof can be varied by varying a stroke of a motor, it is suitably used for a variable cooling controlling.
The construction of a linear compressor in use for the refrigerator or the air-conditioner will now be described.
FIG. 1 is a schematic block diagram of an apparatus for controlling a linear compressor in accordance with a conventional art, which includes a linear oscillating motor 10 for controlling the strength of cooling air by varying a stroke relying on an up/down movement of a piston; an electric circuit unit 20 for controlling an alternate current power source in accordance with a gate control signal so as to control a power supplied to the linear oscillating motor 10; and a control unit 30 for controlling a stroke command value according to inputted temperature information and a stroke estimated by a stroke voltage applied to the linear oscillating motor 10, to be identical to each other, and providing a thusly obtained timer drive signal to the electric circuit unit 20.
The control unit 30 includes a stroke command value determiner 31 for determining a stroke command value corresponding to a temperature upon receipt of the temperature information, and outputting it; a sensorless stroke estimator for receiving stroke voltages V0-V3 provided by the linear oscillating motor, estimating its stroke value and outputting the estimated stroke value; a stroke controller for controlling in a way that the stroke estimated in the sensorless stroke estimator 32 is suitable to the stroke command value determined by the stroke command value determiner 31, and accordingly outputting a timer command value; a zero-cross detector 34 for detecting a zero-cross point from an inputted voltage waveform and outputting a zero-cross signal; and a timer 35 for providing a gate drive signal in accordance with an estimated value estimated by the stroke controller 33 at the time when the zero-cross signal is outputted from the zero-cross detector 34.
The operation of the apparatus for controlling a linear compressor in accordance with a conventional art constructed as described above will now be described.
A power supply voltage as shown in FIG. 2A is applied from a power supply voltage terminal, it is provided to the linear oscillating motor 10 through a current sensing resistance R, a triac Tr and a capacitor C of the electric circuit unit 20, and that way, the current flows to the linear oscillating motor 10. Thereafter, a piston 11 of the linear oscillating motor 10 performs a reciprocal movement, of which reciprocal stroke distance of the piston 11 refers to a stroke. A strength of cooling air can be varied by varying the stroke, that is the strength of cooling air of the refrigerator or the air-conditioner is controlled by varying the stroke.
When a user sets a temperature of the refrigerator or the air-conditioner, information relating to set temperature is received by the stroke command value determiner 31 of the control unit 30. Upon receipt of the temperature information, the stroke command value determiner 31 determines a stroke command value corresponding to the set temperature and provides a signal of thusly determined stroke command value to the stroke controller 33.
At this time, the sensorless stroke estimator 32 receives from the linear oscillating motor 10 the voltage V0 between the current sensing resistance R and the power supply voltage terminal, the voltage V1 between the current sensing resistance R and the triac Tr, the voltage V2 supplied from the triac Tr to the linear oscillating motor 10, and the voltage V3 supplied to the linear oscillating motor 10 through the capacitor C, estimates stroke information and current information, and transmits thusly estimated information to the stroke controller 33.
Thereafter, the stroke controller 33 controls in a manner that the stroke command value determined by the stroke command value determiner 31 to be identical to the estimated stroke value, and transmits the obtained timer command value to the timer 35.
Then, the zero-cross detector 34 receives the voltage V0 between the current sensing resistance R and the power supply voltage terminal, or the voltage V4, the one before passing the capacitor C starting from the power supply voltage terminal to detect a zero-cross point, and provides a detected zero-cross signal to the timer 35.
Then, the timer 35 receives the zero-cross signal to a start terminal thereof. When the zero-cross signal is inputted to the start terminal, the timer 35 sets a time t1 as shown in FIG. 2E according to a timer command value provided by the stroke controller 33.
After the time t1 is set, the timer 35 outputs a gate drive signal to the gate G of the triac Tr of the electric circuit unit 20. In this respect, if the time t1 is short as shown in FIG. 2C, the gate drive signal is set to be short from the time point of the zero-cross as shown in FIG. 2C, so that a large current flows as shown in FIG. 2D, while, if the time t1 is long as shown in FIG. 2E, the gate drive signal is distanced from the zero-cross time point, so that a small current flows as shown in FIG. 2F.
Therefore, as the gate drive signal is outputted to the gate G of the triac Tr of the electric circuit unit 20, the triac Tr is turned on and the current is supplied to the linear oscillating motor 10, and accordingly, the piston of the linear oscillating motor 10 moves upwardly and downwardly, thereby controlling the strength of cooling air of the refrigerator or the air-conditioner.
When the input current is applied as a periodic function, the movement of the piston has the same cycle, which has various shapes according to the pressure of suction and discharge.
FIG. 4 shows one example of it. Assuming that the cycle of the piston is `T`, since the stroke represents a maximum displacement within one cycle, it is defined by the following equation:
S(k).ident.max(x(t)), (k-1/2+L )T.ltoreq.t&lt;(k+1/2+L )T where x (t) is an estimated value by the senseless stroke estimator, there may exist an error between the estimated value and the real value as e(k)=x(k)-x(t).
In case that the linear oscillating motor 10 makes a model as an R-L circuit having a back electromotive force as shown in FIG. 3, a theoretical basis for representing the movement of the piston can be expressed by the following two nonlinear simultaneous differential equation: ##EQU1##
where x indicates a displacement of the piston, i indicates a current flowing to the motor, m indicates a mass of the piston, C indicates a damping coefficient, k indicates an equivalent spring constant, Fp indicates a force applied by the piston, .alpha. indicates a back electromotive force constant, L indicates an equivalent inductance coefficient, R indicates an equivalent resistance, r indicates a resistance for sensing a strength of current (r&lt;&lt;R), and V indicates an external voltage.
Referring to the above equation, Fp represents a force according to a pressure difference between suction and discharge, which is non-linearly varies momently while the compressor passes the suctioning-discharging-suctioning processes.
According to the equation, if the voltage V is increased, the right side of the equation (2) becomes larger, and thus, the current of the left side becomes strong. Then, the right side of the equation (1) becomes larger, and accordingly, the displacement of the piston of left side becomes larger.
That is, the stroke distance of the piston is varied by an applied voltage, and when the triac, a semiconductor switching device, is used, the applied voltage can be controlled by switching, having the same effect.
However, referring to the conventional linear compressor, when a stroke reaches the boundary (discharge valve face), as shown in FIG. 6, the operation of the piston often turns unstable. In other words, the operation of the piston becomes very unstable at the position where the piston very nears the discharge valve and almost collides with the discharge valve.
In addition, referring to the linear compressor, its efficiency is the best at tuning point and noise is the least generated. In this respect, it often occurs that the operation of the piston becomes unstable as shown in FIG. 6. The reason for this has not been revealed. One of assumption is that it may be due to a hysteresis characteristics of an actuator, which is shown in a simulation based on an experiment and the above equations (1) and (2).
The instability of the operation of the piston leads to a problem in that the input power supply is shaken, and the strength of cooling air is accordingly shaken, which is very undesirable for the refrigerator or the air-conditioner adapting the liner compressor. In this respect, however, notably, an optimum operational point can be detected by using the fact that the unstable phenomenon occurs in the tuning point.
In addition, in the conventional linear compressor, a clearance volume needs to be controlled accurately, but due to the characteristics deviation of parts of the complicated sensorless circuit or the deviation between the major mechanic parts inside the compressor, a serious deviation is made even from a desired strength of cooling air under the same stroke control.